Dock, wharf (4)
I believe the answer is:
pier
This clue is a double definition.
'dock' is the first definition.
(I know that dock can be written as pier)
'wharf' is the second definition.
(I know that wharf can be written as pier)
(Other definitions for pier that I've seen before include "Platform built out from the shore, supported on piles" , "Seaside structure" , "Landing-stage or promenade" , "Structure with a deck, built over water" , "Bridge pillar" .)